Gunmen raided the headquarters of a pro-government Syrian TV station early on Wednesday, killing seven employees, kidnapping others and demolishing buildings, officials said.

An Associated Press photographer who visited the al-Ikhbariya station's compound said five portable buildings used for offices and studios had collapsed, with blood on the floor and wooden partitions still on fire. Some walls had bullet holes.

Al-Ikhbariya is privately owned but strongly supports President Bashar Assad's regime. Pro-government journalists have been attacked on several previous occasions during the country's 15-month uprising. "What happened today is a massacre," the information minister, Omran al-Zoebi, told reporters. He blamed terrorists – the same word the government uses for rebels.

Rebels deny they target the media. Much of the violence that has gripped Syria over the past 15 months has been sanctioned by the government to crush dissent. But rebel fighters are launching increasingly deadly attacks on regime targets, and several massive suicide attacks this year suggest al-Qaida or other extremists are joining the fray.

The US secretary of state, Hillary Clinton, said on Wednesday that she has "great hope" that a meeting of key powers on Saturday in Geneva can be a turning point in the Syria crisis. But the UN gave a dire assessment of the crisis, saying the violence has worsened since a ceasefire deal that was supposed to go into effect in April, and the bloodshed appears to be taking on more dangerous, sectarian overtones.

Syria severely restricts the media in the country, making it difficult to gain a credible account of events on the ground. Assad denies that there is any popular will behind the uprising, saying terrorists are behind a conspiracy to destroy the country.

Zoebi said gunmen stormed al-Ikhbariya's compound in the town of Drousha, about 14 miles south of the capital Damascus, and detonated explosives. He said the attackers killed seven people and kidnapped others.

In comments broadcast on state-run Syrian TV, he said the killings amounted to "a massacre against the freedom of the press". Most news organizations in Syria are either state-run or private bodies that carry the government's point of view. Most of the private TV stations and newspapers are owned by politicians or wealthy businessmen who have close links to the regime.

An employee at the station said several other staffers were wounded in the attack, which happened just before 4am local time. He said the gunmen kidnapped him along with several station guards. He was released but the guards were not.

The employee, who did not give his name for fear of repercussions, said the gunmen drove him about 200 metres away, then he heard the explosion from the station being demolished. "I was terrified when they blindfolded me and took me away," the man said by telephone.

Earlier this month, two al-Ikhbariya employees were shot and seriously wounded by gunmen in the north-western town of Haffa while covering clashes between government troops and insurgents. Hours after the attack, the station was still on the air, broadcasting a rally in Damascus's main square against the station raid.

Meanwhile, Burhan Ghalioun, the former leader of Syria's main opposition group, said he briefly entered rebel-held areas in the north of the country in a rare trip by the exiled political opposition to the country. Ghalioun told al-Jazeera TV that the areas he visited in Idlib province are ruling themselves, without any regime presence.

Ghalioun, former head of the Syrian National Council, did not say when the visit happened. "I went to see the war that the Syrian regime is staging," Ghalioun said. "The regime continues to shell and kill." Ghalioun said he spoke with wounded Syrians including some who lost limbs and others who were paralysed. He added that he was able to drive about freely and that "part of the country is liberated".

Activists reported violence throughout Syria on Wednesday. The London-based Syrian Observatory for Human Rights said at least 10 government soldiers were killed in an ambush in the eastern province of Deir el-Zour.

The group said that rebels on Tuesday were able to shoot down a helicopter gunship in Idlib province. Amateur videos showed a helicopter burning in a field but the report could not be independently confirmed.

In neighboring Turkey, some 30 more Syrian soldiers defected with their families overnight, the country's state-run Anadolu news agency reported. It was not clear if the group included any senior officers.

Assad's regime has suffered an embarrassing string of high-ranking defections this week, with dozens of soldiers, including senior officers, reported to have fled to Turkey.